The effect of composition on pressure-induced devitrification in metallic glasses

Due to the lack of long-range translational periodicity of glass, diffraction as the most powerful structure probe for crystalline materials provides very limited information of the glass structure. Thus the structure of glass has been a long-standing mystery. Pressure is a powerful tool for altering individual atoms and electrons, changing the bonding chemistry, and creating novel materials [QS Zeng et al., Proc. Natl. Acad. Sci. U.S.A. 106, 2515 (2009)]. By applying high pressure, we observed a novel single crystal crystallization with fixed crystallographic orientation at room temperature in Ce₇₅Al₂₅ metallic glass for the first time [QS Zeng et al., Science 332, 1404 (2011)]. The experimental and theoretical effort further found the unusual single crystal formation with fixed crystallographic orientation from a glass is caused by the hidden long-range topological order in the glass structure. The pressure-induced devitrification (PID) directly links the “disordered” glass structure with the perfectly ordered single crystal structure, thus provides us a unique approach revealing the otherwise invisible long-range topological order in metallic glasses.

However what compositions may have PID and an understanding of the physical and chemical controls behind PID is still not clear. To address this question, we systematically studied the Ce_xAl_1-x (x=65, 70, and 80 at.%) metallic glasses by using in situ high pressure x-ray diffraction and molecular dynamics (MD) simulations. A strong effect of composition on PID in metallic glass was observed and our elastic instability model indicates the metallic glasses with PID must meet two conditions: (1) the larger atoms are more compressible than the smaller ones (pressure can then decrease the atomic radii difference) and (2) the solute concentration must be located within a narrow region below the saturated critical value under high pressure and above the critical value required for glass formation at ambient pressure. The critical concentration can be quantitatively calculated by ΣC_i*|(r_i/r_a)³-1|≈1, where r_i and r_a are the atomic radii of the solute and solvent respectively. The effect of composition on PID in metallic glasses addressed here provides new insight and guidance for searching for PID in other MG systems, and sheds light on the investigation of some fundamental questions in glass structure by linking the glass structure with crystal structure directly via PID, and the novel materials synthesis from amorphous precursors via PID as well.